In-situ observations of the polymorphic transformation in a single targeted Cu₆Sn₅ grain constrained between Sn-0.7 wt % Cu solder and Cu-Cu₃Sn phases and the associated structural evolution during a solid-state thermal cycle were achieved via a high-voltage transmission electron microscope (HV-TEM) technique. Here, we show that the monoclinic η'-Cu₆Sn₅ superlattice reflections appear in the hexagonal η-Cu₆Sn₅ diffraction pattern upon cooling to isothermal 140 °C from 210 °C. The in-situ real space imaging shows that the η'-Cu₆Sn₅ contrast pattern is initiated at the grain boundary. This method demonstrates a new approach for further understanding the polymorphic transformation behavior on a real solder joint.